1. Field of the Invention
The present invention relates to a field of optical communication, particularly, to an optical transmission line used for a wavelength-division multiplex transmission and an optical transmission system using the optical transmission line.
2. Description of the Related Art
Presently, vigorous studies are being made of the wavelength-division multiplex transmission, and vigorous efforts to develop an optical transmission line using wavelength-division multiplexing transmission are being made. As reported in conference proceedings such as OFC '96 WKI and OFC '97 TuN2, optimization of non-zero dispersion-shifted optical fiber (NZ-DSF) was pursued in the past as such an optical transmission line.
Non-zero dispersion-shifted optical fiber is an optical fiber in which the zero dispersion wavelength is shifted so as to diminish the dispersion at a wavelength in 1.55 μm band, which is being studied nowadays for the wavelength-division multiplexing transmission. The non-zero dispersion-shifted optical fiber that is proposed nowadays has a dispersion of to 8 ps/nm/km at wavelengths in 1.55 μm band.
However, such a non-zero dispersion-shifted optical fiber gives rise to the problem that, if the effective core area (Aeff) of the optical fiber is increased in general in an attempt to suppress the self phase modulation (SPM) or the cross phase modulation (XPM), the bending loss or the dispersion slope is increased. Also, since four-wave mixing (FWM) is prominent in the case where the dispersion is small, the non-zero dispersion-shifted optical fiber in which the dispersion is small in the used wavelength band as described above is disadvantageous in this point.
Under the circumstances, a method for managing the dispersion over the entire optical transmission line is being proposed in order to overcome the problems noted above. For example, Jpn. Pat. Appln. KOKAI Publication No. 8-42102 discloses that it is possible to obtain an optimum transmission line by connecting a positive dispersion optical fiber having a low non-linearity and a negative dispersion optical fiber having a relatively high non-linearity in this order.
Also, as a specific example, an optical transmission line of SMF+RDF, in which a single mode optical fiber (SMF) is connected to a transmission line type dispersion compensation optical fiber, is proposed in, for example, “ECOC '97 Vol. 1, P127”. Since each of SMF and RDF, which has a large local dispersion (absolute value) of about 16 to 22 ps/nm/km at a wavelength in 1.55 μm band, is advantageous in the aspect of suppressing the FWM.
The SMF+RDF optical transmission line is a system in which a single mode optical fiber (SMF) having a dispersion of, for example, 16 to 20 ps/nm/km at a wavelength of 1.55 μm is arranged in the front stage and the dispersion or the dispersion slope of the single mode optical fiber is compensated by a dispersion compensation optical fiber (RDF) connected to the rear stage and having reverse dispersion characteristics. Proposed in the SMF+RDF optical transmission line is the SMF:RDF length ratio falling within a range of between 1:1 and 5:1.
However, where higher speed transmission is considered, it is known in the art that the large-dispersion absolute value of the SMF or RDF generates a large accumulated dispersion in the optical transmission line, with the result that a greatly distorted waveform brings about a mutual function among the signals so as to raise an obstacle to high-speed transmission.
To be more specific, in the optical transmission line of SMF+RDF proposed above, the length ratio of SMF:RDF falls within a range of between 1:1 and 5:1. In the optical transmission line in which the SMF occupies more than half the optical transmission line, the accumulated dispersion is made not smaller than 8×L (ps/nm), where L represents the length of the optical transmission line. This means that, if the length of the optical transmission line is assumed to be, for example, 40 km, the accumulated dispersion is not smaller than 8×40=320 ps/nm, which is disadvantageous in performing high-speed transmission.